The steering systems of highway vehicles and the like are designed primarily for driver control. In these systems, the steering force required on the steering wheel and the ratio between steering wheel movement and movement of the steerable ground wheels depend upon the characteristics of the particular vehicle and the conditions under which it will usually be operated. A wide variety of extraneous forces can act on a vehicle steering system and spurious steering inputs caused by these forces must be dealt with satisfactorily in order to provide stable and controllable steering of a vehicle. As vehicle speed increases, the effects of any spurious steering inputs are magnified, making it necessary for the driver to exercise more precise and careful driving control.
Vehicles with steering systems having positive caster generally track relatively straight ahead and generally resist steering inputs away from center, including those of the driver. Such positive caster is provided by positive caster offset, which is also known as mechanical trail. Caster offset is the distance from the ground intersection point of a line drawn through the pivot axis of a steerable wheel to a point at the center of the area over which the wheel contacts the ground. The pivot axis of a steerable wheel of a motor vehicle is usually provided by a "king pin". Because the contact point of a steerable wheel with positive caster trails the pivot line point of the wheel, side forces cause the wheel to turn in the direction that the force is being applied. A good example of this is the way in which the castered wheels on the front of a shopping cart are easily turned in the direction of applied force.
The adverse effects described below are some of the negative aspects of achieving steering stability through generous amounts of positive caster offset. Because of the side force applied by gravitational pull on a slanted or crowned highway, positive caster offset causes a motor vehicle to freely turn to the low side, creating a steering wheel pull that requires counteractive steering input from the driver to keep the vehicle from leaving the highway. Thus, when driving on a crowned or slanted road, the driver must hold against wheel pull mile after mile. The culprit here is the hundred year old steering geometry that is responsible for the automatic, never failing pull to the low side of the roadway. The amount of driving fatigue that is directly caused by positive caster offset under these conditions may be appreciated by considering the many millions of miles driven by motorists each day on crowned or slanted highways.
Another fatiguing driving condition that may be encountered by a motorist is that of controlling a crosswind steering input. The amount of adverse steering input caused by crosswinds is directly related to the amount of positive caster offset, which is a classic example of having to balance a benefit with a detriment. The small amount of stability gained from castering the steerable wheels on a non-windy day may be paid for many times over when driving in a crosswind because of the destabilizing effect of the crosswind when combined with positive caster offset.
Positive caster offset also allows steering inputs from rutted and other imperfect roadway surfaces to steer back against the driver and thereby cause road wander, which is a universal driving complaint, particularly by driver's of heavy vehicles such as trucks and motor homes. Road wander occurs when the steered wheels do not hold a dedicated straight line. It also may be caused by wear or slack in the steering system and, where power steering is used, because of dead space travel of the power steering valve before it opens the fluid ports of the power steering unit. When the steered wheels wander, the problem is passed on to the driver who must make constant steering corrections.
Because of their high-roll center dynamics, trucks, buses, motorhomes and other large vehicles are especially susceptible to a condition known as "dynamic sway". Dynamic sway is similar to road wander and is caused by steered wheels that will not hold a straight line because of vehicle geometry. Attempts to solve this steering problem have universally been made by making the chassis stiffer in the roll axis. However, there are practical limits to how rigid the roll axis can be made.
Another drawback of prior art steering systems is that spurious inputs transmitted from the roadway through the steerable wheels affect substantially the entire steering assembly because the only stabilizing resistance is provided by the driver's manipulation of the steering wheel. The transmission of these various inputs between the steerable wheels and the steering wheel causes steering system oscillations and unnecessary component wear.
The ideal driving situation is therefore one where the steering system inherently causes the vehicle to travel in an unswerving straight line unless the driver intentionally turns the vehicle in another direction. Thus, the ideal steering system would require relatively little attention from the driver as the vehicle progresses along a straight line path down the roadway. From a steering standpoint, the vehicle should not respond to anything but the driver's steering commands and these must be of sufficient magnitude to overcome a significant resistance to turning away from center. In the absence of a steering input by the driver, the vehicle should literally do nothing but progress straight ahead.